Most power factor correction techniques incorporate a boost topology, which is operated in either continuous or discontinuous inductor current mode and is operated at a fixed or variable switching frequency. Due to a lower peak current, the continuous inductor current mode, which operated at a fixed switching frequency, is used for higher power applications. For lower power applications, the discontinuous inductor current mode, which is operated in a variable switching frequency, provides several advantages including smaller inductor size, lower costs, simpler circuitry, and zero current switching (ZCS).
FIG. 1 illustrates a conventional PFC converter, in which a switching signal VG is connected to a transistor 10 to switch an inductor 20 and to control an input current IIN. An input current IAC of the PFC converter is controlled to achieve a lower current harmonic distortion.
FIGS. 2A and 2B illustrate the input current waveforms IAC and IIN in response to the input voltages VAC and VIN for the conventional PFC converter. The pulse width of the PFC controller is controlled by a voltage error amplifier, which is compared to a saw-tooth waveform generated by a control circuit. The pulse width varies with the line and load conditions but should be maintained at a constant for a half line cycle. Therefore the voltage error amplifier is necessary to have a lower frequency bandwidth that is below the line frequency. The ZCS includes several application advantages. For example, the inductor current is released to zero before the next switching cycle is started thus producing higher switching efficiency. Because the change of the inductor current is equal to the peak inductor current and the current starts and returns to zero on each cycle, the current waveform has a triangular shape with an average value equal to one-half of the peak current multiplied by its time. Since ZCS is switched right on edge between continuous and discontinuous current modes; therefore, variable switching frequency is resulted. The low-bandwidth pulse width modulation (PWM) incorporating ZCS provides a natural power factor correction for an input current.
FIG. 3 illustrates the input current (IIN) waveform, which is increased in response to the enabling of the switching signal (VG), for the conventional PFC converter. The on-time (TON) and the off-time (TOFF) represent a charge period and a discharge period of the inductor 20, respectively.
FIGS. 4A, 4B, and 4C illustrate the three control stages T1–T3 of the conventional PFC converter. The inductor 20 is charged when the transistor 10 is turned on. The energy of the inductor 20 is discharged to the capacitor 50 through a rectifier 30 once the transistor 10 is turned off. The output voltage (VO) of the PFC converter is normally set to a higher voltage, such as 400V, to achieve a better power factor control. Therefore, a parasitic capacitor 15 of the transistor 10 shall be charged up to the higher voltage VO during the discharge period of the inductor 20. As illustrated in FIG. 4C, the energy stored in the parasitic capacitor 15 shall be discharged to the capacitor 16 (or a parasitic capacitor) after the inductor 20 is fully discharged and before the transistor 10 is turned on. A voltage VOS is thus produced on the capacitor 16. The voltage VOS therefore inhibits the input current IAC which flows through the bridge rectifier 40 during the lower VAC period.
FIGS. 5A and 5B illustrate the input current distortion that is caused by the voltage VOS. Recently a variety of discontinuous current PFC controllers have been developed for the power factor control, such as ST6561 of ST-Microelectronics, France; and TDA4862 of Siemens, Germany.
FIG. 6A illustrates the circuit schematic of the aforementioned conventional PFC controllers, in which a multiplier terminal (VM) is connected to sense the input waveform VIN via a plurality of resistors 21 and 22. The voltage sensed on the multiplier terminal (VM) is applied to modulate the on-time (TON) as illustrated in FIG. 6B. The modulated on-time (TON) shall reduce the voltage VOS and improve the input current waveform. However, the drawbacks of the foregoing approach are a higher power consumption of the resistor 21 and complexity of the control circuitry. In addition, another drawback of the aforementioned controllers is without under-voltage protection, which causes the PFC converter to overload in case of brownout conditions.